[Hallicrafters] Of Variacs

[john]

I use a variac, not so much to reform caps (I disconnect the load and use 
an external PS to reform them), but to carefully monitor powerup current 
while bringing up an unknown piece of gear.

By the way, reforming is not a band aid...it's a normal part of the 
technology of Aluminum Electrolytics.  All AlEl caps exhibit the 
"reforming"  phenomenon..even relatively new ones. When doing Incoming 
Inspection, it's normal to allow the caps a short time to reform, and grow 
the oxide layer when measuring leakage.  OTOH, if the caps don't come up 
pretty quickly, I just replace them.

BTW, I just finished restoring a HT17 that had been a very nice unit (with 
the origninal pilot crystal, instead of an aftermarket meter!) before being 
beat up badly due to poor packing. The cabinet and chassis were both bent, 
enough to have broken one of the ceramic ends of the load control variable 
cap. After getting all of that damage repaired, I found that all three 
electrolytics in the can type PS filter cap were virtually shorted. I've 
never seen such bad caps.... all three!

I removed the old guts, installed 3 new Nichicon replacements in the old 
can, reassembled and it's now all back on the air.  This was advertised as 
a radio "my radio man has checked this as working".  He must have checked 
it with a set of calipers, I guess!


 >The only advantage a Variac would give you (after you've replaced the
 >electrolytics!)  is to lower the line voltage if yours is above 117-
 >120V -- and there's some debate about whether that's necessary

While " necessary" may be up for debate,  the effect of over voltaged 
filaments on tube lifetime is not debatable. The physics of  wearout 
mechanisms of a vacuum tube are well known. Over voltage on the filament 
DOES reduce tube lifetime in a significant way.  I'm not fanatical about 
maintaining filament voltage on most receiving equipment, but I sure am on 
the expensive PA tubes in my amps.

Cheers.....
John K5MO



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